Lab: Decomposition Reaction
Introduction:
The decomposition of a compound into simpler substances by means of an electric current is called
electrolysis. During the electrolysis of water, electricity is passed through water, which then separates into its
elemental components of oxygen and hydrogen.
Since pure water does not conduct electricity well enough to allow electrolysis to occur a reasonable rate, a
catalyst is needed to speed up the reaction. In this experiment, sodium carbonate in solution will be used to
increase the rate of the reaction. The sodium hydroxide ionizes in water, much like NaCl, making the solution
an electrolyte.
Pre-lab Questions:
1. What are the chemical formulas for water, hydrogen, and oxygen?
2. Using these formulas, write and balance the chemical equation for the electrolysis (decomposition) of
water including states of matter.
3. Using your chemical equation, how much hydrogen gas would you expect as compared with oxygen gas?
Why?
4. On the picture of the setup below, label the test tube in which you predict to collect the oxygen gas and the
test tube in which you predict you will be collecting the hydrogen gas. What allows you to make these
predictions? Red wire is positively charged; Black wire is negatively charged)
Setup:
Electrolysis Apparatus
Procedure:
1. Fill a 250 mL beaker with tap water until the height (approximately 200 mL) is above the ends of the
electrodes.
2. Fill both test tubes with a 0.25 M solution of sodium hydroxide (NaOH) and cap them with the provided
rubber stoppers, so that there is no air trapped inside the test tubes.
3. Turn them over into the beaker so that the stoppered end is below the level of the water.
4. Use a spatula to remove the stoppers allowing them to fall to the bottom of the cylinder. Leave the stoppers
on the bottom of the beaker.
Reason: The stoppers will be needed to recap the test tubes after the electrolysis.
5. Work the test tubes over the electrodes without losing the solution.
6. Plug in the battery eliminator with the power turned off. Plug the end of the black lead into the black knob of
the battery eliminator and the end of the red lead into the red knob of the battery eliminator. Explanation:
The battery eliminator is a direct current power supply, it operates like a plugged in battery.
7. Clamp one alligator clip to electrode A and the other to B.
8. Now switch the apparatus on (the little red light should light up) and adjust the voltage so that it is between
9 and 12 volts.
9. Observe the apparatus and answer some of the lab questions until one of the tubes completely fills with
gas.
10. Switch off and unplug the battery eliminator.
11. Use the spatula to position the stoppers so that a test tube can be lowered onto them. Without losing any
gas, lower the test tubes onto the stoppers.
12. Remove the stoppered test tubes and place them right side up in a test tube rack or small beaker.
13. Testing the gas: Working with your partner, test the smaller quantity of gas with a glowing wood splint or
toothpick.
Caution: Point the opening of the test tube away from yourself, your partner, and everyone else when
conducting this test.
14. Test the larger quantity of gas by placing a flaming toothpick to the opening immediately after removing the
stopper.
Reason: If you do not do this immediately, the gas will escape and no reaction will be visible. Caution:
Point the opening of the test tube away from yourself, your partner, and everyone else when conducting
this test.
Analysis Questions:
1. Complete the table below.
Electrode
How much of the test tube is filled?
(½, ¾, completely full)
Identity of the gas
Red
(positive)
Black
(negative)
2. Charged particles were needed to make the reaction faster. What ions were present in the solution due to
the sodium hydroxide?
3. Energy was put into the reaction to break water apart into hydrogen and oxygen. What evidence showed
that energy is released when water is formed from the hydrogen?